1. Field of the Invention
The disclosed invention relates to semiconductor devices including semiconductor elements and methods for manufacturing the semiconductor devices.
2. Description of the Related Art
Storage devices including semiconductor elements are broadly classified into two categories: volatile storage devices that lose stored data when power is not supplied and nonvolatile storage devices that retain stored data even when power is not supplied.
A typical example of a volatile storage device is a dynamic random access memory (a DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is accumulated in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost according to the principle; thus, another writing operation is necessary every time data is read. Further, a transistor included in a storage element has leakage current (off-state current) or the like between a source and a drain in an off state and electric charge flows into or out even if the transistor is not selected, so that a data retention period is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power is not supplied, a different storage device including a magnetic material or an optical material is needed in order to retain data for a long time.
A different example of a volatile storage device is a static random access memory (an SRAM). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation, which is an advantage over a DRAM. However, cost per storage capacity is high because a circuit such as a flip-flop is used. Further, as in a DRAM, stored data in an SRAM is lost when power is not supplied.
A typical example of a nonvolatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region of a transistor and stores data by holding electric charge in the floating gate. Thus, a flash memory has advantages in that a data retention period is extremely long (semi-permanent) and refresh operation which is necessary in a volatile storage device is not needed (for example, see Patent Document 1).
However, there is a problem in that a storage element does not function after a predetermined number of writing operations because a gate insulating layer included in the storage element is degraded by tunneling current generated in writing operations. In order to reduce adverse effects of this problem, a method for equalizing the number of writing operations among storage elements is employed, for example. However, this method needs a complex peripheral circuit. Further, even when such a method is employed, fundamental life problems cannot be solved. That is, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary in order to inject electric charge in a floating gate or to remove the electric charge, and a circuit for generating high voltage is required. Further, it takes a comparatively long time to inject or remove electric charge, and it is not easy to increase the speed of writing or erasing data.